While I was away at the SEPnet meeting yesterday a story broke in the press broke about the discovery of a large underdensity in the distribution of galaxies. The discovery is described in a paper by Szapudi et al. in the journal Monthly Notices of the Royal Astronomical Society. The claim is that this structure in the galaxy distribution can account for the apresence of a mysterious cold spot in the cosmic microwave background, shown here (circled) in the map generated by Planck:

We use the WISE-2MASS infrared galaxy catalogue matched with Pan-STARRS1 (PS1) galaxies to search for a supervoid in the direction of the cosmic microwave background (CMB) cold spot (CS). Our imaging catalogue has median redshift z ≃ 0.14, and we obtain photometric redshifts from PS1 optical colours to create a tomographic map of the galaxy distribution. The radial profile centred on the CS shows a large low-density region, extending over tens of degrees. Motivated by previous CMB results, we test for underdensities within two angular radii, 5°, and 15°. The counts in photometric redshift bins show significantly low densities at high detection significance, ≳5σ and ≳6σ, respectively, for the two fiducial radii. The line-of-sight position of the deepest region of the void is z ≃ 0.15–0.25. Our data, combined with an earlier measurement by Granett, Szapudi & Neyrinck, are consistent with a large Rvoid = (220 ± 50) h−1 Mpc supervoid with δm ≃ −0.14 ± 0.04 centred at z = 0.22 ± 0.03. Such a supervoid, constituting at least a ≃3.3σ fluctuation in a Gaussian distribution of the Λ cold dark matter model, is a plausible cause for the CS.

The result is not entirely new: it has been discussed at various conferences over the past year or so (e.g this one) but this is the first refereed paper showing details of the discovery.

Anyway, I just wanted to make a few points about this because some of the press coverage has been rather misleading. I’ve therefore filed this one in the category Astrophype.

First, the “supervoid” structure that has been discovered is not a “void”, which would be a region completely empty of galaxies. As the paper makes clear it is less dramatic than that: it’s basically an underdensity of around 14% in the density of galaxies. It is (perhaps) the largest underdensity yet found on such a large scale – though that depends very much on how you define a void – but it is not in itself inconsistent with the standard cosmological framework. Such large underdensities are expected to be rare, but rare things do occur if you survey a large enough volume of the universe. Large overdensities also arise as statistical fluctuations in large volumes.

Second, and probably most importantly, although this “supervoid” is in the direction of the CMB Cold Spot it cannot on its own explain the Cold Spot; the claim in the abstract that it provides a plausible explanation of the cold spot is simply incorrect. A void can affect the measured temperature of the CMB through the Integrated Sachs-Wolfe effect: photons travelling through such a structure are redshifted as they travel through the underdense region, so the CMB looks cooler in the direction of the void. However, even optimistic calculations of the magnitude of the effect suggest that this particular “void” can only account for about 10% of the signal associated with the Cold Spot. This is a reasonably significant contribution but it does not account for the signal on its own.

This is not to say however that it is irrelevant. It could well be that the supervoid actually sits in front of a region of the CMB sky that was already cold, as a result of a primordial fluctuation rather than a line-of-sight effect. Such an effect could well arise by chance, at least with some probability. If the original perturbation were a “3σ” temperature fluctuation then the additional effect of the supervoid would turn it into a 3.3σ effect. Since this pushes the event further out into the tail of the probability distribution it makes a reasonably uncommon feature look less probable. Because the tail of a Gaussian distribution drops off very quickly this has quite a large effect on the probability. For example, a fluctuation of 3.3σ or greater has a probability of 0.00048 whereas one of 3.0σ has a probability of 0.00135, about a factor of 2.8 larger. That’s an effect, but not a large one.

In summary, I think the discovery of this large underdensity is indeed interesting but it is not a plausible explanation for the CMB Cold Spot. Not, that is, unless there’s some new physical process involved in the propagation of light that we don’t yet understand.

As you will no doubt be aware, tomorrow there will be a Partial Eclipse of the Sun visible from the United Kingdom. Here’s a handy guide, courtesy of the Met Office, to the time and maximum fraction of the Sun’s disk that will be obscured.

Unfortunately the weather forecast for Brighton isn’t marvellous so it’s possible that the main event will be obscured by cloud and all we experience is that an already dark and gloomy morning gets even darker and gloomier.

However, in the event that the weather forecast turns out to be inaccurate, which is far from unheard of, please make sure you follow the official Royal Astronomical Society guidelines to make sure you observe it safely.

And while I’m at it, here is a video of a nice lecture by Ian Ridpath explaining all about Solar and Lunar Eclipses.

As a spectacle a partial solar eclipse is pretty exciting – as long as it’s not cloudy – but even a full view of one can’t really be compared with the awesome event that is a total eclipse. I’m lucky enough to have observed one and I can tell you it was truly awe-inspiring.

If you think about it, though, it’s rather odd that such a thing is possible at all. In a total eclipse, the Moon passes between the Earth and the Sun in such a way that it exactly covers the Solar disk. In order for this to happen the apparent angular size of the Moon (as seen from Earth) has to be almost exactly the same as that of the Sun (as seen from Earth). This involves a strange coincidence: the Moon is small (about 1740 km in radius) but very close to the Earth in astronomical terms (about 400,000 km away). The Sun, on the other hand, is both enormously large (radius 700,000 km) and enormously distant (approx. 150,000,000 km). The ratio of radius to distance from Earth of these objects is almost identical at the point of a a total eclipse, so the apparent disk of the Moon almost exactly fits over that of the Sun. Why is this so?

The simple answer is that it is just a coincidence. There seems no particular physical reason why the geometry of the Earth-Moon-Sun system should have turned out this way. Moreover, the system is not static. The tides raised by the Moon on the Earth lead to frictional heating and a loss of orbital energy. The Moon’s orbit is therefore moving slowly outwards from the Earth. I’m not going to tell you exactly how quickly this happens, as it is one of the questions I set my students in the module Astrophysical Concepts I’ll be starting in a few weeks, but eventually the Earth-Moon distance will be too large for total eclipses of the Sun by the Moon to be possible on Earth, although partial and annular eclipses may still be possible.

It seems therefore that we just happen to be living at the right place at the right time to see total eclipses. Perhaps there are other inhabited moonless planets whose inhabitants will never see one. Future inhabitants of Earth will have to content themselves with watching eclipse clips on Youtube.

Things may be more complicated than this though. I’ve heard it argued that the existence of a moon reasonably close to the Earth may have helped the evolution of terrestrial life. The argument – as far as I understand it – is that life presumably began in the oceans, then amphibious forms evolved in tidal margins of some sort wherein conditions favoured both aquatic and land-dwelling creatures. Only then did life fully emerge from the seas and begin to live on land. If it is the case that the existence of significant tides is necessary for life to complete the transition from oceans to solid ground, then maybe the Moon played a key role in the evolution of dinosaurs, mammals, and even ourselves.

I’m not sure I’m convinced of this argument because, although the Moon is the dominant source of the Earth’s tides, it is not overwhelmingly so. The effect of the Sun is also considerable, only a factor of three smaller than the Moon. So maybe the Sun could have done the job on its own. I don’t know.

That’s not really the point of this post, however. What I wanted to comment on is that astronomers generally don’t question the interpretation of the occurence of total eclipses as simply a coincidence. Eclipses just are. There are no doubt many other planets where they aren’t. We’re special in that we live somewhere where something apparently unlikely happens. But this isn’t important because eclipses aren’t really all that significant in cosmic terms, other than that the law of physics allow them.

On the other hand astronomers (and many other people) do make a big deal of the fact that life exists in the Universe. Given what we know about fundamental physics and biology – which admittedly isn’t very much – this also seems unlikely. Perhaps there are many other worlds without life, so the Earth is special once again. Others argue that the existence of life is so unlikely that special provision must have been made to make it possible.

Before I find myself falling into the black hole marked “Anthropic Principle” let me just say that I don’t see the existence of life (including human life) as being of any greater significance than that of a total eclipse. Both phenomena are (subjectively) interesting to humans, both are contingent on particular circumstances, and both will no doubt cease to occur at some point in perhaps not-too-distant the future. Neither tells us much about the true nature of the Universe.

Perhaps we should just face up to the fact that we’re just not very significant….

Well, it’s official that this afternoon’s announcement of a “major discovery” is going to be from the BICEP team, and it specifically concerns the BICEP2 CMB telescope experiment. I’ve just got back to Sussex (after a weekend in Cardiff) and will be following the events in among other things I have to do before going off to give a lecture at 5pm GMT.

The schedule of events is as follows: there will be a special webcast presenting the first results from the BICEP2 CMB telescope. The webcast will begin with a presentation for scientists 10:45-11:30 EDT, followed by a news conference 12:00-1:00 EDT.

14:48 Straight to the headline: R=0.2 (+0.07, -0.05) with R=0 rejected at about 7 sigma, if you like things stated in such terms…

14:53 Here’s the crucial graph. Results a bit higher than the expected signal at l in range 200-300?

15:06 The news avalanche has started, e.g. here at the BBC, but there is some concern about the shape of the spectrum.

15:10 I’m not getting anything from the press conference, so may have missed important details. It seems to me though that there’s a significant possibility of some of the polarization signal in E and B not being cosmological. This is a very interesting result, but I’d prefer to reserve judgement until it is confirmed by other experiments.

15:35 Despite the press hype there’s still some scepticism among cosmologists arising from the strange-looking shape of the spectrum. I’m not convinced myself. Anyway, I have to sign off now in order to prepare a lecture..

16:20 Back-of-the-envelope time: if the result is correct then the inflationary energy scale is about 2×1016 GeV. That’s just two orders of magnitude below the Planck scale…

18:19 Returned from my 5pm Theoretical Physics lecture. Couldn’t resist spending 30 minutes talking about BICEP2, though I did tell them it’s not in the examination.

18:25 Main points of controversy:

there seems to be evidence of leakage of temperature into polarization (lines in Fig. 5);

there’s an excess in the B-B spectrum at l~250 shown above;

there’s an excess at low l in the E-E spectrum

there’s a deficit at low l in the cross-correlation with Keck

There may be a connection between 1. and 2.-4. If 2.-4 are real then they may be evidence of something interesting that requires more than a straightforward modification of inflation (such as might include just a running of the spectral index).

18:35 Other controversy: why has this result been announced before the paper has been published or even peer-reviewed?

Well, in case you hadn’t noticed, the cosmology rumour mill has gone into overdrive this weekend primarily concerning the possibility that an experiment known as BICEP (an acronym formed from Background Imaging of Cosmic Extragalactic Polarization). These rumours have been circulating since it was announced last week that the Harvard-Smithsonian Center for Astrophysics (CfA) will host a press conference on Monday, March 17th, to announce “a major discovery”. The grapevine is full of possibilities, but it seems fairly clear that the “major discovery” is related to one of the most exciting challenges facing the current generation of cosmologists, namely to locate in the pattern of fluctuations in the cosmic microwave background evidence for the primordial gravitational waves predicted by models of the Universe that involve inflation.

Anyway, I thought I’d add a bit of background on here to help those interested make sense of whatever is announced on Monday evening.

Looking only at the temperature variation across the sky, it is not possible to distinguish between tensor (gravitational wave) and scalar (density wave) contributions (both of which are predicted to be excited during the inflationary epoch). However, scattering of photons off electrons is expected to leave the radiation slightly polarized (at the level of a few percent). This gives us additional information in the form of the polarization angle at each point on the sky and this extra clue should, in principle, enable us to disentangle the tensor and scalar components.

The polarization signal can be decomposed into two basic types depending on whether the pattern has odd or even parity, as shown in the nice diagram (from a paper by James Bartlett)

The top row shows the E-mode (which look the same when reflected in a mirror and can be produced by either scalar or tensor modes) and the bottom shows the B-mode (which have a definite handedness that changes when mirror-reflected and which can’t be generated by scalar modes because they can’t have odd parity).

The B-mode is therefore (at least in principle) a clean diagnostic of the presence of gravitational waves in the early Universe. Unfortunately, however, the B-mode is predicted to be very small, about 100 times smaller than the E-mode, and foreground contamination is likely to be a very serious issue for any experiment trying to detect it. To be convinced that what is being measured is cosmological rather than some sort of contaminant one would have to see the signal repeated across a range of different wavelengths.

Moreover, primordial gravitational waves are not the only way that a cosmological B-mode signal could be generated. Less than a year ago, a paper appeared on the arXiv by Hanson et al. from SPTpol, an experiment which aims to measure the polarization of the cosmic microwave background using the South Pole Telescope. The principal result of this paper was to demonstrate a convincing detection of the so-called “B-mode” of polarization from gravitational lensing of the microwave background photons as they pass through the gravitational field generated by the matter distributed through the Universe. Gravitational lensing can produce the same kind of shearing effect that gravitational waves generate, so it’s important to separate this “line-of-sight” effect from truly primordial signals.

So we wait with bated breath to see exactly what is announced on Monday. In particular, it will be extremely interesting to see whether the new results from BICEP are consistent with the recently published conclusions from Planck. Although Planck has not yet released the analysis of its own polarization data, analysis of the temperature fluctuations yields a (somewhat model-dependent) conclusion that the ratio of tensor to scalar contributions to the CMB pattern is no more than about 11 per cent, usually phrased in the terms, i.e. R<0.11. A quick (and possibly inaccurate) back-of-the-envelope calculation using the published expected sensitivity of BICEP suggests that if they have made a detection it might be above that limit. That would be really interesting because it might indicate that something is going on which is not consistent with the standard framework. The limits on R arising from temperature studies alone assume that both scalar and tensor perturbations are generated by a relatively simple inflationary model belonging to a class in which there is a direct relationship between the relative amplitudes of the two modes (and the shape of the perturbation spectrum). So far everything we have learned from CMB analysis is broadly consistent with this simplifying assumption being correct. Are we about to see evidence that the early Universe was more complex than we thought? We'll just have to wait and see…

Incidentally, once upon a time there was a British experiment called Clover (involving the Universities of Cardiff, Oxford, Cambridge and Manchester) which was designed to detect the primordial B-mode signal from its vantage point in Chile. I won’t describe it in more detail here, for reasons which will become obvious.

The chance to get involved in a high-profile cosmological experiment was one of the reasons I moved to Cardiff in 2007, and I was looking forward to seeing the data arriving for analysis. Although I’m primarily a theorist, I have some experience in advanced statistical methods that might have been useful in analysing the output. Unfortunately, however, none of that actually happened. Because of its budget crisis, and despite the fact that it had spent a large amount (£4.5M) on it already, STFC decided to withdraw the funding needed to complete it (£2.5M) and cancelled the Clover experiment. Had it gone ahead it would probably have had two years’ data in the bag by now.

It wasn’t clear that Clover would have won the race to detect the B-mode cosmological polarization, but it’s a real shame it was withdrawn as a non-starter. C’est la vie.

Last week there was a rather tedious flurry of media activity about Stephen Hawking’s alleged claim that there are no black holes after all. Here’s a nice blog post explaining what Hawking actually said. Also, check out the link at the start of this article to a very nice layperson’s guide to the Black Hole Information Paradox.

First, Hawking does not have a new theory… at least not one he’s presented. You can look at his paper here — two pages (pdf), a short commentary that he gave to experts in August 2013 and wrote up as a little document — and you can see it has no equations at all. That means it doesn’t qualify as a theory. “Theory”, in physics, means: a set of equations that can be used to make predictions for physical processes in a real or imaginary world. When we talk about Einstein’s theory of relativity, we’re talking about equations. Compare just the look and…

I recently came across a post by distinguished astrophysicist Scott Tremaine who works at the Institute for Advanced Study in Princeton. The piece is entitled “Overblown Statements in Press Releases Undermine Science”, something that exercised me so much that I invented the category Astrohype so I could post particularly egregious examples on this blog.

Soctt Tremaine’s piece is on the American Astronomical Society website, but I’m reposting the text here to give it wider circulation as I think it makes some very important points that we’d all do well to heed. And of course in the interest of full disclosure I should point out that I am a theoretical astrophysicist myself, so may be a bit biased…

–o–

In a recent column, AAS President David Helfand argued correctly that negative public messages about subfields within our own discipline, or even about other disciplines — “shooting inward at each other” — damage all of us.

Consider, then, the following public messages:

from a major research university, a press release titled “Astronomers Discover Planet that Shouldn’t Be There,”

from the National Radio Astronomy Observatory, a press release containing the quote, “Much of what we thought we understood about the physics of pulsars and neutron stars may be wrong,”

from the Space Telescope Science Institute, a press release stating, “New observations from NASA’s Hubble Space Telescope challenge 30 years of scientific theory about quasars,” and

from a respected news organization, an interview with a prominent exoplanet researcher containing the quote, “Theory has struck out.”

The point is not whether these messages provide accurate characterizations of the state of theoretical understanding in their respective subject areas (though in most cases they do not). The point is that by belittling and trivializing the efforts of theoretical astrophysicists — who try to understand extremely complex processes in exotic environments, with limited clues from observations — they damage the public perception of the entire astronomy community. As just one example, statements from press releases such as those above are often repeated on creationist websites, where they carry extra weight because they have the imprimatur of NASA or a major observatory or university.

Advances in observational astronomy are spectacular enough to appeal to the public on their own merits, without “shooting inward” at efforts to understand these observations. Astronomers and press officers can provide a more realistic picture of the synergy between observation and theory, and in so doing would improve the public perception of astronomy research in particular and of the scientific enterprise more generally.

Anyway, since the paper I found is a review article I hoped it would help teach me the error of my ways. Here is the abstract

This article discusses density perturbations in inflationary models, offering a pedagogical description of how these perturbations are generated by quantum fluctuations in the early universe. A key feature of inflation is that that rapid expansion can stretch microscopic fluctuations to cosmological proportions. I discuss also another important conseqence of quantum fluctuations: the fact that almost all inflationary models become eternal, so that once inflation starts, it never stops.

My eye was drawn to the phrase “almost all inflationary models”. I had hoped to see “almost all” used in its strict mathematical sense, ie “apart from a set of measure zero” with the measure being fully specified. Disappointingly, it isn’t. Guth discusses the consequences of the tail the inflationary potential V (for large values of the inflaton field ϕ) on the long-term evolution of inflationary dynamics and then states

Since V3/2/|V ′| grows without bound as ϕ → ∞ for most potentials under consideration, almost all models allow for eternal inflation.

This means, to me, most models people have constructed but doesn’t mean all possible models. I don’t doubt that some inflationary models become eternal, but would have preferred a more rigorous statement. This is particularly strange because Guth spends the last section of his paper discussing the “measure problem”:

While the multiverse picture looks very plausible in the context of inflationary cosmology — at least to me — it raises a thorny and unsolved problem, known as the “measure problem.” Specifically, we do not know how to define probabilities in the multiverse.

The measure problem to my mind also extends to the space of all possible inflationary theories.

And then there’s the title, which, I remind you, is Quantum Fluctuations in Cosmology and How They Lead to a Multiverse. Guth’s argument consists of going through the (standard) calculation of the spectrum of cosmological density fluctuations (which does fit a host of observational data). He then states:

Since the density perturbation calculations have been incredibly successful, it seems to make sense to take seriously the assumptions behind these calculations, and follow them where they lead. I have to admit that there is no clear consensus among cosmologists, but to many of us the assumptions seem to be pointing to eternal inflation, and the multiverse.

I have to admit that I get a bit annoyed when I read a paper in which the actual conclusions are much weaker than implied by the title, but that seems to be par for the course in this field.

For the record, I’ll state that I am an agnostic about the multiverse. It may be a correct idea, it may not. I will say, however, that I still haven’t found any article that puts it on a firm scientific footing. That may well, of course, just be a measure of my ignorance. If you know of one, please let me know through the comments box.

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